Over the past two decades, the use of composite materials has become increasingly widespread. Fiber-reinforced polymeric composites make up the bulk of the composite materials that are used, particularly in low temperature and structural applications. The processing of composite materials is typically applied to objects having a relatively simple geometry--primarily surfaces and tubes. Fabrication of more complex shapes from composites generally requires secondary joining and/or machining processes and subsequent assembly of component parts, which introduce complexity tending to both limit potential applications and increase the concomitant costs.
For high temperature applications, metal matrix composites (MMCs) offer improved mechanical properties and significant mass reduction compared to conventional metal alloys. One of the most common forms of MMCs is an aluminum alloy that is reinforced with silicon carbide particles. This type of MMC is becoming more readily available in both wrought and foundry grade remelt billets. However, the range of available shapes for wrought materials of this type is again limited to relatively simple shapes that are common to standard rolling and extrusion practices; complex designs require additional manufacturing processes and joining of subassemblies. In contrast, high quality MMC components can be readily produced at relatively low cost using a casting process.
A wide range of casting techniques are currently incorporated for the production of particulate or short fiber reinforced composites, including spray processes, dispersion processes, and pressure impregnation. In the spray process, a gas atomized liquid matrix is co-sprayed with reinforcing dispersoids onto a mandrel or substrate to form a consolidated cast composite part or layer. This process lends itself to the manufacture of tubes and other shapes of revolution, or flat stock. Dispersion processes include stir and compo-casting, and screw extrusion. In the stir and compo-casting of composites, particulates or short fibers are mechanically mixed with a liquid or partially crystallized slurry and then introduced to a shaped mold. The screw extrusion process uses a screw extruder to act as both a mechanical mixer and a viscosity pump to produce extruded shapes or billets.
Pressure impregnation processes include pressure infiltration and squeeze casting. Both processes rely on gas pressure differentials or mechanical pressure to impregnate a preform of consolidated particulates or short fibers with a matrix material. Generally, each of the aforementioned types of processes may be applied to either polymer or metal matrix material systems.
Composite materials with increasing levels of reinforcement typically have unique casting limitations that limit their use in more conventional casting processes. Each of the casting processes noted above represents an attempt to overcome some of these limitations, but all are, in general, more complicated and expensive than standard single alloy casting practices.
One of the most significant casting limitations of composite materials is the reduction of fluidity that results from the presence of high volume fractions of reinforcing particulates or short fibers. The low fluidity of a liquid composite material greatly restricts the distance that the material may flow into a typical casting mold and the size of the minimum cross section through which the material may pass. This limited fluidity can severely limit the allowable complexity and detail of the part being cast, as well as limit the volume fraction of reinforcement used.
Another important casting limitation of composite materials relates to the difficulty of producing a homogenous mixture of reinforcing particulates or short fibers, and a matrix material. Typically, the reinforcing particulates or short fibers are mixed with the matrix material prior to casting by mechanical means. When reinforcement particulates are introduced into a fluid matrix and homogenized by a mechanical mixing head, a by-product of the mixing process is the creation of gas bubbles that survive in the cast product, causing undesirable porosity. The presence of this porosity in the final solidified part can adversely affect its overall mechanical properties to an unacceptable degree.
In the case of MMCs, the mixing of reinforcing particulates or short fibers into the matrix prior to casting, at temperatures above the liquidus temperature of the matrix material, increases the likelihood of the formation of reaction by-products of the reinforcement and matrix materials. For example, aluminum carbide is readily formed at the interface between SiC particulates and an aluminum alloy matrix at elevated melt temperatures. For most high temperature composite systems, the reaction byproduct that is formed is an intermetallic material that typically exhibits brittle mechanical behavior. The presence of a brittle interface between the reinforcing particulate and the matrix can lead to a significant decrease in tensile, fracture, and fatigue properties of a cast MMC part, as well as further reduction in the overall fluidity of the mix during the casting process. To overcome some of the problems associated with the low fluidity of the composite melt, volume fraction of the reinforcement particulates or short fibers added to the matrix is typically limited to a relatively low level, e.g., from 3 to 20 percent by volume, thereby limiting the material property improvement that can be achieved by the addition of the particulates or short fibers to the composite.
Within the past five years, a family of foundry grade particulate reinforced MMCs produced by the dispersion process have become increasingly available. The most common of these is an aluminum-silicon alloy matrix that is homogeneously mixed with discontinuous silicon carbide particulates. Volume loading of reinforcement ranges typically between 5 and 20 percent. At the higher levels of particulate loading, the tensile properties and coefficient of thermal expansion can approach those of cast irons. With strengths that are similar to those of cast irons, a comparable part can be produced with less than one half the mass, with upwards of three times the thermal conductivity and significantly increased abrasion resistance. The properties of this family of materials suggests a wide range of commercial applications; especially in areas where weight reduction is advantageous.
From the perspective of Metallurgical Engineering, net shape casting practices offer one of the most versatile means of controlling microstructure and material property development. This statement is especially true for particulate reinforced MMCs, where the type and loading of the reinforcement material chosen has a distinct influence on the overall material properties. This family of composites have the potential of allowing the engineering of a specific part to meet a range of mechanical requirements through control of reinforcement loading within the piece, as well as the selection of the matrix alloy employed. This type of reinforcement loading control has typically been achieved through the use of reinforcement preforms or by powder metallurgy processes, though neither of these manufacturing routes lend themselves to the economies of general foundry practice.
Clearly, it would be desirable to provide a new process that permits the production of complex cast particulate MMC parts with controlled gradients of reinforcement, suitable to the standard foundry environment. The cost or complexity of currently available approaches to produce complex parts from composite materials does not offer any acceptable solutions to this problem.